What is the real focal length of the RedCat 51?

William Optics state 250 mm at f/4.9. Plate solves of my Dumbbell Nebula and Hercules Cluster stacks put it at 248 mm, 3.12 arcseconds per pixel with 3.76 micron pixels.

What field of view does the RedCat 51 give?

5.4 x 3.6 degrees on an APS-C sensor such as the ZWO ASI2600MC: enough for the whole Veil Nebula in one frame.

Does the RedCat 51 need a field flattener?

No. It is a four-element Petzval with a 48 mm image circle. William Optics recommend 55 mm of back spacing, plus or minus 3 mm.

How do you focus the RedCat 51?

With the knob of the internal rack and pinion focuser, which has 23 mm of travel inside the tube and takes a ZWO EAF, and the Bahtinov mask built into the lens cap.

RedCat 51 or Askar SQA55?

I think the SQA55 beats the RedCat: a 55 mm f/4.8 quintuplet Petzval at 264 mm with a full-frame image circle, £795 against £849. Its one downside is the helical focuser, which takes more fiddling to set up with an autofocuser.

How much does the RedCat 51 cost and where can I buy it?

The prices I recorded: £849 at First Light Optics (UK), in stock; $878 at Agena Astro (US), in stock; $798 at William Optics (Global), in stock.

What have I not tested on the RedCat 51?

I have not used the RedCat 51 visually, on the Moon, the planets or the Sun, on a full-frame camera, or measured its star shapes across the field.

William Optics RedCat 51 review: the fan-favourite Petzval

Where does it rank on our leaderboard

Nine of the 42 telescopes on my board, against price
Rank	Telescope	Stars for Your Buck	Recorded UK price
1	Askar 50P	95 out of 100	£299
2	ToupTek Hope D60	93 out of 100	£749
7	Askar SQA85	88 out of 100	£2,395
16	SVBONY SV555	79 out of 100	£509
29	William Optics RedCat 51	69 out of 100	£849
32	Sky-Watcher HAC125	61 out of 100	£405
38	Altair 60EDF	51 out of 100	£499
41	Celestron 114 LCM	42 out of 100	£361
42	Celestron AstroMaster LT 76	13 out of 100	£200

Contents

01 Where does it rank on our leaderboard
02 Specifications and features
03 In the hand
04 How good is it? Here's what I captured
05 Raw against finished
06 The rig on the mount
07 What is it for
08 Watch the review
09 Tested numbers and the download
10 Should you buy it?
11 Questions and more about this review

In 30 seconds

Stars for Your Buck 69/100

Reviewed 18 Jul 2024 Updated 23 Sep 2026

Rank: #29 of 42 telescopes
Verdict: Worth a look

Written by Damon Scotting 5 min read

Buy it if

You want a compact 250 mm Petzval that carries a cooled camera, filter wheel and guide scope on a 5 kg mount.
You want easy focusing: an internal rack and pinion focuser that takes a ZWO EAF, and a Bahtinov mask in the lens cap.
You image large nebulae: on an APS-C camera it frames the whole Veil Nebula.

Skip it if

You want the best 50 mm astrograph for the money: the Askar SQA55 is my pick.
You are on a tight budget: the Askar 50P does the same wide-field job for £299.
You want small galaxies or planetary nebulae large in the frame: 250 mm is too short.

“It's a good telescope, but whenever I show people my gear, this is always their favourite, and I think a large reason for that is because it's unique.”

Specifications and features

Against the telescopes I have reviewed

Compare with

William Optics RedCat 51 reviewed #29 of 42	Askar SQA55 #12 of 42	ToupTek Hope D60 #2 of 42	SVBONY SV555 #16 of 42	Askar 50P #1 of 42	Askar SQA60 Pro #26 of 42
Recorded UK price	£849	£795	£749	£509	£299	£1,249
Stars for Your Buck	69/100	82/100	93/100	79/100	95/100	71/100
Aperture	51 mm	55 mm	60 mm	54 mm	50 mm	60 mm
Focal length	250 mm	264 mm	280 mm	243 mm	190 mm	276 mm
Focal ratio	f/4.9	f/4.8	f/4.66	f/4.5	f/3.8	f/4.6
Optical design	Four-element Petzval, FPL-53 glass	Quintuplet Petzval, SD and ED elements	Five element planar apochromat, built-in flattener	Petzval, 5 elements in 3 groups	Quadruplet Petzval, one SD element	Sextuplet Petzval APO, two SD elements
Image circle	48 mm	44 mm	44 mm	44 mm	APS-C	44 mm
Back focus	55 mm, plus or minus 3 mm	55 mm	55 mm	55 mm, M48x0.75 male thread	55 mm	41 to 66 mm (55 recommended); flattener built in
Focuser	Internal rack and pinion (WIFD), 23 mm travel, ZWO EAF ready	Helical, separate coarse and fine controls	3 inch dual-speed, 30 mm travel, rotator	Helical, lockable 360 degree rotator	Single-speed focus ring, 15 mm travel, rotator	2.8 inch rigid dual speed, 25 mm travel, 360 degree rotator
Weight, tube	1.77 kg; 2.38 kg with ring, handle and dovetail	1.84 kg tube; 2.18 kg with handle and dovetail plate	About 3.5 kg as supplied, not a bare-tube figure	2.45 kg (net)	1.4 kg	2.79 kg tube; 3.54 kg with the ring and dovetail
Length, tube	298 mm	316 mm with dew shield	255 mm	not recorded	147 mm	283 mm
Dew shield	84 mm diameter	Removable	Retractable, magnetic	Reversible lens hood, 93 mm	Retractable	Fixed
In the box	Bahtinov mask cap, ring, handle, 210 mm Vixen and Arca dovetail, tilt adjuster, rotator, M48 adapter, soft case	Tube, waterproof hard case	Tube ring, dovetail, handle, M48 adapter, tilt adjuster, caps	EAF mount kit, handle with Vixen saddle, dovetail, camera angle adjuster, 2 inch filter holder, lens cover, padded case	Tube with ring and dovetail, manual	Tube, tube ring, 290 mm Vixen dovetail, handle, two finder shoes, Oxford cloth case

In the hand

How good is it? Here's what I captured

Three targets

North America Nebula (NGC 7000), ASI533MM Pro, SHO narrowband

Integration: 5 h (180 s subs)
Field: 3.7 x 3.7 degrees
Telescope: William Optics RedCat 51
Camera: ZWO ASI533MM Pro
Mount: Unknown
Target: North America Nebula (NGC 7000)
Type: Emission nebula
Location: Bortle 6.8 (in my garden)

Veil Nebula (NGC 6960 and NGC 6992), ASI2600MC

Integration: 21 min (21 x 60 s)
Field: 5.3 x 3.6 degrees
Telescope: William Optics RedCat 51
Camera: ZWO ASI2600MC
Mount: ZWO AM5
Target: Veil Nebula (NGC 6960 and NGC 6992)
Type: Supernova remnant
Location: Bortle 6.8 (in my garden)

Dumbbell Nebula (M27), ASI2600MC

Integration: 9 min (9 x 60 s)
Field: 5.4 x 3.6 degrees
Telescope: William Optics RedCat 51
Camera: ZWO ASI2600MC
Mount: Unknown
Target: Dumbbell Nebula (M27)
Type: Planetary nebula
Location: Bortle 6.8 (in my garden)

Raw against finished

The rig on the mount

The 2 targets, the raw combined stack against my finished image, each with its integration, the camera and the palette:

North America Nebula (NGC 7000): 5 h (180 s subs) of light, ASI533MM Pro, SHO narrowband, the raw combined stack with one automatic stretch
Dumbbell Nebula (M27): 9 min (9 x 60 s) of light, ASI2600MC, the raw combined stack with one automatic stretch

On the TeSeek 11, then on six other mounts

What is it for

Four things it is for

Focus inside the tube

The patented William Optics internal focus design lets me fine-tune my view of the night sky and achieve pinpoint stars.

A Bahtinov mask in the cap

Better yet, there is an inbuilt Bahtinov mask in the telescope cap, and it comes included with the telescope.

No back focus to calculate

Because of its unique design I do not have to worry about calculating back focus for my camera, which is a huge time saver.

Room for a full imaging rig

The RedCat still has room to attach accessories like a finderscope. My narrowband rig on it, with an ASI533MM Pro, a 7 x 2 inch filter wheel, a 50 mm guide scope and an ASI120MM, weighs 4.11 kg.

Watch the review

The SUPER POWERFUL REDCAT 51 Telescope! Tiny, portable and sharper than ever! #Telescope #Astronomy · published 18 July 2024 · 0:58 · Watch on YouTube · 28,140 views, 911 likes on 2 Oct 2026

Jump to

0:07 The RedCat 51 version 3
0:11 The internal focuser
0:17 A Bahtinov mask in the cap
0:22 Petzval design at f/4.9
0:31 How much it weighs
0:36 No back focus to calculate

Check out more videos on the William Optics RedCat 51

I tried a £299 Telescope mount?! Surprisingly GOOD! (TeSeek 11 mount)

30 Aug 2025

Why it is BETTER than most Mounts? | AM5N Review.

7 Nov 2024

Tested numbers and the download

Finished image

Licence: for personal use, credit Damon Scotting. Both JPEGs 2,495 x 2,476 px.

-10 C sensor temperature during imaging set point -10 C

248 mm focal length, plate solved William Optics states 250 mm

5.42 x 3.62° field of view 6,248 x 4,176 px on ASI2600MC

26 MP ASI2600MC 6,248 x 4,176 px, 23.5 x 15.7 mm

Should you buy it?

69/100 Stars for Your Buck

Pros

The William Optics internal focuser lets me fine-tune focus to pinpoint stars, and it takes a ZWO EAF.
A Bahtinov mask is built into the lens cap, so the focusing aid comes in the box.
No back focus to work out: because of its unique design I do not have to worry about calculating it for my camera, and the Petzval optics need no separate flattener.
Four-element Petzval optics at a fast f/4.9: my stars have never looked rounder, and my plate solves put it at 248 mm against the stated 250 mm.
“Small enough for a small mount: my narrowband rig on it, with camera, filter wheel, guide scope and guide camera, weighed 4.11 kg.”
“A 5.4 x 3.6 degree field on APS-C takes in the whole Veil Nebula in one frame.”

Cons

“I think the Askar SQA55 definitely beats it and therefore establishes itself as the go-to telescope of the 50 mm range.”
At £849 it costs more than the Askar SQA55 (£795), the ToupTek Hope D60 (£749) and the SVBONY SV555 (£509).
It is a bit of a chunky one: 2.38 kg with its ring, handle and dovetail, against 2.18 kg for the SQA55 and 1.4 kg for the Askar 50P.
250 mm is short for small targets: the Dumbbell Nebula is a small disc in my frame.

I actually really like my RedCat.

Who the RedCat 51 is for

Travelling imager

Buy

It is 298 mm long, comes with a soft case, and my full narrowband rig on it weighed 4.11 kg.

Owner of a small harmonic mount

Buy

My RedCat rig with a cooled camera, filter wheel, 50 mm guide scope and guide camera weighed 4.11 kg, inside the 5 kg payload of the TeSeek 11 it rode on.

Wide-field nebula imager

Buy

On an APS-C sensor it frames 5.4 x 3.6 degrees: the whole Veil Nebula fits in one frame.

Budget-conscious beginner

Skip

My recommendation is the Askar 50P: a 50 mm Petzval for £299 that sits at the top of my telescope board.

Galaxy and planetary nebula imager

Skip

At 250 mm the Dumbbell Nebula is a small disc; a longer refractor such as the Askar 80ED at 560 mm suits small targets.

Owner of an older RedCat 51

Skip

Do not upgrade for image quality: the version 3 keeps the same optics as the earlier RedCats, and the new part is the internal focuser.

Three alternatives I have reviewed

Questions and more about this review

Reviewed by

Damon Scotting

The Telescope Man Verified

BSc (Hons) Physics with Astrophysics and Cosmology, Loughborough University PGCE / QTS, University of Nottingham

About Damon About Damon Damon Scotting is The Telescope Man. He holds a BSc (Hons) in Physics with Astrophysics and Cosmology from Loughborough University and a PGCE with QTS from the University of Nottingham, and taught physics in secondary schools before becoming an astronomy creator. He is a contributing writer for BBC Sky at Night Magazine and the author of 42 Wonders of Our Night Sky, and his photograph of the galaxy M100 with the dwarf planet Ceres was Highly Commended in the Astronomy Photographer of the Year 2024 competition run by Royal Observatory Greenwich. He has appeared on the BBC's Blue Peter. More than 750,000 people follow his astronomy channels on YouTube, Instagram and TikTok, and he has reviewed more than 140 telescopes, mounts, cameras, filters and accessories hands-on for this site. More about Damon · How I test · Damon's book

I review all kinds of gear: most of it I buy, some is gifted and some is loaned, and all of it is reviewed the same way. How this site is funded

Eight questions, answered

Questions about the William Optics RedCat 51

Reviewed 18 July 2024 ·Updated 23 September 2026

I review all kinds of gear. Most of it I buy, some is gifted and some is loaned, and all of it is reviewed the same way: no company sees or approves a review before it is published. Shop links marked Ad are affiliate links: I earn a commission when you buy through them, at no extra cost to you. How this site is funded

What is the real focal length of the RedCat 51? William Optics state 250 mm at f/4.9. Plate solves of my Dumbbell Nebula and Hercules Cluster stacks put it at 248 mm, 3.12 arcseconds per pixel with 3.76 micron pixels. Tested numbers and the download ↓
What field of view does the RedCat 51 give? 5.4 x 3.6 degrees on an APS-C sensor such as the ZWO ASI2600MC: enough for the whole Veil Nebula in one frame. Tested numbers and the download ↓
Does the RedCat 51 need a field flattener? No. It is a four-element Petzval with a 48 mm image circle. William Optics recommend 55 mm of back spacing, plus or minus 3 mm. The specifications and the comparison table ↓
How heavy is the RedCat 51? 1.77 kg for the tube and 2.38 kg with its ring, handle and dovetail. My narrowband rig on it, with an ASI533MM Pro, a 7 x 2 inch filter wheel, a 50 mm guide scope and an ASI120MM, weighed 4.11 kg. The rig on the mount ↓
How do you focus the RedCat 51? With the knob of the internal rack and pinion focuser, which has 23 mm of travel inside the tube and takes a ZWO EAF, and the Bahtinov mask built into the lens cap. What is it for ↓
RedCat 51 or Askar SQA55? I think the SQA55 beats the RedCat: a 55 mm f/4.8 quintuplet Petzval at 264 mm with a full-frame image circle, £795 against £849. Its one downside is the helical focuser, which takes more fiddling to set up with an autofocuser. The specifications and the comparison table ↓
How much does the RedCat 51 cost and where can I buy it? The prices I recorded: £849 at First Light Optics (UK), in stock; $878 at Agena Astro (US), in stock; $798 at William Optics (Global), in stock. A record, not a live price The verdict, the readers and the sellers ↓
What have I not tested on the RedCat 51? I have not used the RedCat 51 visually, on the Moon, the planets or the Sun, on a full-frame camera, or measured its star shapes across the field.